1. Field of the Invention
The invention relates in general to a method of fabricating a shallow isolation trench isolation (STI), and more particularly, to a method of fabricating a shallow isolation trench isolation and mending micro-scratch caused by chemical mechanical polishing (CMP).
2. Description of the Related Art
FIG. 1A to FIG. 1D shows a conventional method of fabricating a shallow trench isolation. In FIG. 1A, on a semiconductor substrate 10, a pad oxide layer 12 and a silicon oxide layer 14 are formed sequentially. The silicon nitride 14 and the pad oxide layer 12 are defined and denoted as 14a and 12a to expose a part of the substrate 10. The exposed substrate 10 is etched to formed a trench 16 as shown in FIG. 1B.
In FIG. 1C, an oxide layer 18 is formed over the substrate 10 and fills the trench 16. Using the silicon nitride layer 14a as an etch stop, the oxide layer 18 is etched back by chemical mechanical polishing. A shallow trench isolation 18a is thus formed.
Chemical mechanical polishing removes an outstanding part of a deposition layer by applying chemical agent, that is, slurry to provide a chemical reaction, and by introducing a mechanical force to the chip on the polishing table. More specifically, the removing material on a chip is polished away by the reaction between the polishing pad on the polishing table and very abrasive particles in the slurry.
However, during polishing, a scratch or damage may be caused on the soft material by the particles. In the step planarizing an oxide layer 18 of chemical mechanical polishing for fabricating a shallow trench isolation 18a, since the oxide layer is soft, a micro-scratch 19 is easily formed on the surface thereof. Though the micro-scratch 19 is too fine to be observed by eyes, without mending the micro-scratch 19, a leakage current is introduced either by short of gates which is caused during defining a poly-silicon gate. The reliability and the yield of the circuit is therefore degraded.
Conventional, the micro-scratch is mended by two methods. One is to over etch a gate formed in the subsequent process. While over etching the gate, the gate material in the micro-scratch within the STI is etched and removed, so that the short through the gate material in the micro-scratch is eliminated. The drawback of this method is that the oxide layer in the source/drain region is etched simultaneously to cause a damage to the source/drain region. The other method is to improve the chemical mechanical polishing process. However, the chemical mechanical polishing is too difficult to control without the formation of a micro-scratch.